Conventional printers that print image with color materials (toner, ink, and the like) in the colors cyan, magenta, yellow, and black (CMYK) have been capable of reducing the achromatic (gray) component produced when mixing the colors C, M, and Y and replacing this component with the K color material.
One reason for doing this is that the density of the toner or other color material often fluctuates in laser printers. These fluctuations in density often result in the gray component having a colored tint when gray is printed using a mixture of C, M, and Y color materials. The gray can be printed with greater stability by using a color conversion profile that gives priority to the K color material.
However, when using a color conversion profile that emphasizes the K color material, the amount of CMYK color materials used for colors approaching gray are found through interpolation based on set values for gray and colors near gray. As a result, the K color material is often used for rendering dark flesh tones and dark blues, but the K color material gives the flesh tones and blues a noticeable grainy appearance.
On the other hand, while a profile giving priority to C, M, and Y color materials reduces the grainy appearance of flesh tones and blues, the gray tones often take on a color tint, as described above. Therefore, it is difficult to obtain satisfactory printing results when the image to be printed includes a combination of colors with high saturation and colors with low saturation.
For example, a method of creating a color conversion table for converting the input image data to image data for printing on an inkjet printer is known, the color conversion table being designed to increase the amount of black ink generated as the saturation increases and to decrease the amount of black ink generated as the saturation decreases.
However, when using a single color conversion table to vary the priority of the K color material based on the amount of color saturation, it is difficult to apply optimum conversion values at points found through interpolation.
Specifically, it is very unrealistic to provide this type of color conversion table with density conversion values for all input colors. For example, if the input values are RGB values, each having 256 possible color levels, the color conversion table would have to store color conversion values for 256×256×256 colors. Since this is unrealistic, color conversion tables are generally provided with density conversion values only at lattice points (reference points) for 17×17×17 colors spaced at intervals of sixteen color levels, while density conversion values for colors that do not correspond to these lattice points are found through interpolation.
Hence, even when optimum conversion values are set for these lattice points, there is no guarantee that interpolation will render optimum conversion values for points other than the lattice points. For example, it is conceivable that more K color material than necessary will be used because a lattice point giving priority to the K color material is one of the lattice points used for interpolation, even when it would be normally desirable to use less K color material.